1. Field of the Invention
The present invention relates to implantable medical devices and methods, and more particularly to an implantable pacemaker that records the occurrence and rate of specified pacemaker events in sequence as such events occur, either on an every event or a sampled basis. Such sequence of events and their respective rates, when retrieved from the pacemaker, is useful as a diagnostic tool for analyzing the performance of the pacemaker and for aiding in optimally programming the pacemaker for a particular patient.
A pacemaker is an implantable medical device that delivers electrical stimulation pulses to a patient's heart, as required, in order to keep the heart beating at a desired rate. Early pacemakers provided stimulation pulses at a fixed rate or frequency, such as 70 pulses per minute (ppm), thereby maintaining the heart beat at that fixed rate. Subsequently, pacemakers were designed to not only stimulate the heart, but also to monitor the heart. If a natural heart beat was detected within a prescribed time period (usually referred to as the "escape interval"), no stimulation pulse was delivered, thereby allowing the heart to beat on its own without consuming the limited power of the pacemaker. Such pacemakers are referred to as "demand pacemakers" because stimulation pulses are provided only as demanded by the heart.
Early demand pacemakers had a fixed base rate associated therewith. In later versions, the base rate was programmably selectable, and thereafter became commonly known as the "programmed rate." If the heart was able to beat on its own at a rate exceeding the base (or programmed) rate, then no stimulation pulses were provided. However, if the heart was not able to beat on its own at a rate exceeding the base rate, then stimulation pulses were provided to ensure that the heart would always beat at least at the base (or programmed) rate.
In recent years, rate-responsive pacemakers have been developed that automatically change the rate at which the pacemaker provides stimulation pulses as a function of a sensed parameter (physiological or other). The sensed parameter provides some indication of whether the heart should beat faster or slower, depending upon the needs of the pacemaker user. Thus, for example, if a patient is at rest, there is generally no need for a faster-than-normal heart rate, so the rate-responsive pacemaker maintains the pacing rate at a normal value, such as 60 ppm. However, if the patent is exercising, or otherwise physiologically active, there is a need for the heart to beat much faster, such as 100 beats per minute. For some patients, the heart is not able to beat faster on its own, so the pacemaker must assist. In order to do this effectively, the physiological need for the heart to beat faster must first be sensed, and the pacing rate of the rate-responsive pacer must be adjusted accordingly. Hence, rate-responsive pacemakers are known in the art as devices that increase and decrease the "base rate" as a function of sensed need (physiological or other).
For purposes of the present application, the following definitions are provided, which are intended as generic definitions that may be used for either single- or dual-chamber pacing. Single-chamber pacing relates to monitoring and/or pacing within only one chamber of the heart, the atrium or the ventricle. Dual-chamber pacing relates to monitoring and/or pacing in both chambers of the heart. A "P-wave" is an electrical signal manifest when the atrium contracts naturally. An "R-wave" is an electrical signal manifest when the ventricle contracts naturally. An "A-pulse" is an electrical stimulation pulse that is generated by the pacemaker and delivered to the atrium in order to force an atrial contraction. A "V-pulse" is an electrical stimulation pulse that is generated by the pacemaker and delivered to the ventricle in order to force a ventricular contraction. A "cardiac cycle" is the time it takes the heart to cycle through one complete sequence of atrial and ventricular contractions. A "pacing cycle" is a cardiac cycle as measured or determined within the pacemaker. The cardiac or pacing cycle may be measured between any two recurring points or events associated with such sequence of contractions, such as P-waves, R-waves, A-pulses, V-pulses, or combinations thereof. The cardiac cycle may be represented by an A-pulse followed by a V-pulse extending to the next A-pulse or P-wave, by an A-pulse followed by an R-wave extending to the next A-pulse or P-wave, by a P-wave followed by a V-pulse extending to the next A-pulse or P-wave, or by a P-wave followed by an R-wave extending to the next A-pulse or P-wave.
Continuing with the definition of terms, a "PVE" is a premature ventricular event, i.e., an R-wave that occurs without a sensed intervening P-wave or A-pulse. The "MTR" is the maximum tracking rate of the pacemaker, and defines a minimum time interval that must elapse after a specified ventricular event, e.g., an R-wave or a V-pulse, before another paced ventricular event (V-pulse) is allowed to take place in response to a sensed P-wave. The "MSR" is the maximum sensor rate, and is used in conjunction with rate-responsive pacemakers to define the maximum pacing rate that the pacemaker may assume under sensor drive. Both the MTR and the MSR are parameters used to limit the maximum rate at which a pacemaker is allowed to provide stimulation pulses on demand. These need not be the same rate.
In addition to the above terms, it should also be noted that modern programmable pacemakers may be programmed to operate in several different modes. By convention, operating modes are designated with a three or four letter code. The first letter of the code signifies the chambers of the heart where pacing may occur, with "A" signifying the atrium; "V" the ventricle; "O" none; and "D" both the atrium and ventricle or dual chamber. The second letter of the code signifies the chambers of the heart where sensing may occur, using the same letters. The third letter of the code signifies the type of action taken by the pacemaker in response to a native complex (P- or R-wave), with "O" being asynchronous pacing-namely, no sensing; "I" indicating that the stimulation pulse is inhibited if sensing occurs; "T" indicating that a stimulation pulse is delivered or triggered in response to a sensed P- or R-wave (useful for diagnostic purposes); and "D" indicating both inhibiting or triggering functions. In a DDD pacemaker, a sensed P-wave inhibits the atrial output pulse but triggers a ventricular output after a preset delay (the AV interval). If an R-wave is sensed before release of the atrial pulse, it inhibits and resets both the atrial and ventricular pulses. If an R-wave is sensed within the alert period of the AV delay, it inhibits the ventricular stimulus and resets the pacemaker. If an R-wave is sensed during a special detection window occurring shortly after an atrial stimulus, it triggers a ventricular output pulse. A fourth letter signifies the extent of programmability, communicating or telemetric features, and rate-responsive features of the pacemaker, if any. An "R" is used as the fourth letter to indicate a rate-responsive pacemaker with the sensor programmed ON (meaning that the sensor is enabled to detect physiological or other activity and is connected to the pacemaker circuits so as to adjust the pacemaker's pacing rate as a function of such sensed physiological or other activity). An "O" in the fourth position means that the pacemaker is not capable of being adjusted (programmed) noninvasively and does not have rate-modulated capability. A "P" means that one or two parameters can be programmed or adjusted noninvasively. An "M" in the fourth position means that the pacemaker has three or more parameters which can be programmed. "C" means the device has "M" programming capability and can communicate with the programmer. Again "R" means rate-modulated capability; it also reflects "C" capability as well.
Nearly all implantable pacemakers in use today, as well as similar implantable medical devices, can be configured by the attending physician in the physician's office. The process of configuring a pacemaker is commonly referred to as "programming." The programming process uses noninvasive telemetry to customize the operation of the pacemaker to fit the individual needs of the patient. Customization is achieved by adjusting a set of "pacemaker parameters" to values that cause the pacemaker to work in an optimum way for the particular patient within whom the device has been implanted.
Disadvantageously, as the complexity of new implantable devices has evolved over the past several years, it has become increasingly difficult for the attending physician, or other medical personnel, to determine how the pacemaker should be programmed in order to provide the most effective therapy for a given patient. This difficulty is particularly manifest with recent-generation pacemakers that tend to be more automatic and autonomous than earlier-generation pacemakers, which recent-generation pacemakers may be controlled by input signals received from a multiplicity of internal sensors. For example, recent "rate-responsive" pacemakers, as indicated above, provide stimulation pulses to a patient's heart, as needed, based on the input signals received from one or more physiological or other sensors that attempt to predict just how fast the patient's heart should beat in order to meet the patient's physiological needs.
A significant factor that makes the optimum programming of recent-generation pacemakers more difficult is the variation in each of the sensor inputs from patient to patient. Such variation is caused by numerous factors, including the patient's physical structure, the implant site, the particular disease or malady the patient has and its progression within the patient's heart or other body tissue, the drugs being taken by the patient to treat his or her condition, etc. Thus, to appropriately program the pacemaker for a given patient, the physician must anticipate how the pacemaker will operate given all of these variables, and given all the environments and activities that the patient is expected to encounter. Programming a modern pacemaker may thus comprise an extremely formidable task, for which task there is a critical need for programming aids to assist the physician in anticipating the pacemaker response for each particular patient.
It is known in the art to use programming aids and devices with implantable pacemakers to facilitate the physician's understanding of the pacemaker's programmed operation as it interacts with the patient's natural cardiac activity. For many years, the primary programming aid and source of diagnostic data for use in analyzing the operation of an implanted pacemaker has been the surface electrocardiogram (ECG), in which both pacemaker and heart activity are recorded and displayed simultaneously. From the ECG, the activity of the heart --including the contraction of the atria, the contraction of the ventricles, and the timing therebetween--could be displayed. From the pacemaker, the activity of the pacemaker--including when a heart contraction was sensed and when a stimulation pulse was generated--could likewise be monitored through the use of marker signals telemetered from the pacemaker to a remote (non-implanted) receiver, where such signals were processed and displayed as marks superimposed on the ECG waveform.
In recent years, specific programming devices have been developed that not only allow the pacemaker parameters to be noninvasively set to desired values, but that also allow the operation of the pacemaker and the heart to be monitored without having to obtain a surface ECG. Such is accomplished by transmitting an intercardiac ECG signal, either alone or in combination with marker signals. See, e.g., U.S. Pat. Nos. 4,559,947; 4,596,255; 4,791,936; and 4,809,697.
Disadvantageously, while such prior art programming devices have done much to facilitate communications with and analysis of implantable programmable pacemakers, they all suffer from one major drawback--they are limited to real-time data analysis. This is true even though some provide the capability of capturing a short segment, e.g., 30 seconds, of the intracardiac ECG signal, which intracardiac ECG signal, once captured, can advantageously be expanded, compressed, or otherwise processed in a desired manner in order to better examine it. Unfortunately, in order to properly assess some types of problems that may develop for a given patient having an implanted pacemaker, it is frequently necessary to examine the intracardiac signal, or at least the main components thereof, over a much longer period of time, e.g., minutes, hours, days, weeks, or months. What is needed, therefore, is not only a programming device that facilitates the physician's ability to understand the interaction of the implanted device with the patient and to evaluate active clinical problems, but a device that provides the physician with sufficient data to allow the performance of the system to be assessed over an extended period of time.
Moreover, in some instances, it may be necessary, or at least desirable if a proper analysis of a particular problem is to be made, to carefully examine selected cardiac signals at a particular moment in time, which particular moment in time may have occurred at a time prior to the evaluation. Such analysis could reveal, for example, whether a PVC occurred, or if a particular cardiac cycle was made up of paced events or sensed events, or both. Unfortunately, there are no known devices that conveniently provide such past information with sufficient detail to enable such a careful analysis to be made. Hence, there is also a need for a device that allows the past interaction between a pacemaker and a patient's heart to be carefully examined and studied.
The commonly used solution to the above-described problem is to use a Holter Monitor, or equivalent external device. A Holter Monitor is essentially a recorder that is carried by the patient. The Holter Monitor senses the surface ECG signal and records it. Thus, after the data-collection period during which the Holter Monitor is used (usually 24 hours), the number of specific cardiac events that occurred, e.g., the number of ventricular contractions, may be determined.
Disadvantageously, Holter Monitors, and equivalent devices, are external devices that must be carried by the patient continuously throughout the monitoring period. Such carrying can be a nuisance and a bother to the patient. Further, such externally-carried recording devices suffer from numerous limitations. One of the main limitations is their inability to consistently identify the high-frequency pacing stimulus generated by the pacemaker. This is particularly the case when the pacemaker generates a bipolar pacing pulse, which pacing pulse is of relatively low amplitude and difficult to detect (compared to a unipolar pacing pulse). Thus, although the contraction of the heart may be recordable, there is no way of easily determining whether the contraction was a natural contraction or a paced contraction. Further, if a pacing pulse is generated and the heart does not respond to it (i.e., if there is a lack of "capture"), such lack-of-capture event may not be detectable.
Another limitation of Holter Monitors, and equivalent externally-carried recording devices, is that they have no way of monitoring, and hence recording, the internal state of the implanted pacemaker. A knowledge of the internal state of a pacemaker would be invaluable in determining the actual pacemaker behavior. Advantageously, the implanted pacemaker "knows" exactly when it paces and senses on each channel and how it is responding to every input signal. Thus, what is needed is an implanted pacemaker that is equipped to track and report its behavior over time; and, upon command, provide such information to an attending physician.
There are at least two prior art devices known to the inventors that attempt to address at least some of the above needs. On such device is the Cosmos.TM. 283-01 pacemaker marketed by Intermedics, Inc. of Angleton, Tex. The Cosmos 283-01, using a feature termed "Diagnostic Data.TM.," counts the number of specific cardiac events that occur during the monitoring period. See, Sanders et al., "Data Storage and Retrieval by Implantable Pacemakers for Diagnostic Purposes," PACE Vol. 7, pp. 1228-33 (1984); and Levine, P. A., "Diagnostic Data: An Aid to the Follow-Up and Assessment of the Pacing System," Journal of Electrophysiology, Vol. 1, pp. 144-53 (1987). The other device is the CHORUS.TM. 6033 pacemaker marketed by ELA Medical, of Montrouge, France. Both of these devices, and similar devices described in the literature, see, e.g., U.S. Pat. No. 4,513,743 (van Arragon et al.), are thus able to determine the total number of occurrences of a selected cardiac event, e.g., the number of times that an atrial pulse is issued; or the number of times that an atrial-to-ventricular (AV) interval has a delay that falls within defined time limits. Knowing the total number of occurrences of such events is a helpful diagnostic tool in assessing some pacemaker/patient problems. Disadvantageously, however, such devices do not record the frequency ranges (rate) of the cardiac events that are counted. Nor do they record the sequence in which the cardiac events occur, or the time at which the cardiac events occurred. Such limitations prevent numerous evaluations from being performed, such as the composition (sensed or paced events) of a prior sequence of cardiac cycles under stressed conditions, the variations in the heart rate over a past period of time, the determination of sensor responsiveness, or an assessment of chronotropic competence. What is needed, therefore, is an implantable medical device that not only detects and records the occurrence of specified cardiac events, and in particular pacemaker events or states, but that also determines and records the sequence of such events and the rate of occurrence of such events.
The present invention advantageously addresses the above and other needs.